A principal disadvantage of prior art projection systems is that they heat, age and melt film, slides, liquid crystal cells and even optical fiber ends. Many thermal control methods are presently used to reduce the heat to protect such media. A most effective method for controlling media temperature is shown in the present inventor's U.S. Pat. No. 5,099,399, which uses a cylindrical glass coupler between the lamp and the graphics medium. A reflector lamp focuses a very intense spot of light on a proximal (inner) end of the glass coupler. The glass rod absorbs the infrared heat and conducts it out of the light beam. The coupler described in the '399 patent has a tuned length to diameter aspect ratio, whereby very intense and sharply-focussed, light enters the proximal end of the coupler, but the light emitted from the other end is cold and uniformly distributed across the distal (outer) end glass surface. As a result, after continuous operation of a 150-watt lamp, one can touch the end of the glass rod and feel no heat. This has proven highly successful in use since 1991, and it is so efficient that 150-watt projector according to the '399 patent produces more usable light than most 250-watt projectors.
Another disadvantage of prior art projectors, including movie projectors, video projectors and fiber optics projectors is that they produce light in a circular beam, but the graphics media used, such as movie film, projection slides, liquid crystal cells or imaging fiber optic arrays, are all rectangular. The transition from the circular lamp beam to a rectangular medium format is currently accomplished by using a rectangular mask to shape the light beam before it strikes the mask. The mask can be a simple planar aperture, or it can be a rectangular glass coupler, such as in U.S. Pat. No. 5,341,445, which uses a polygonal coupler instead of the cylindrical coupler of the '399 Patent. Polygonal masks are optically inefficient, as the edges of the circular lamp beam are clipped off, allowing only the rectangular portion of the circular beam to pass into the mask and onto the graphics medium.
Although the inventor's '399 patent solved the media heating problem, there is still another limitation on projected beam brightness and overall optical efficiency. That limitation is the numerical aperture mismatch between the projection lamp and the imaging medium.
A projection lamp has a very large aperture, near F:1, to capture and focus as much of the source radiation as possible. An aperture of f:1 means the distance from the lamp reflector face to the focal plane, where the source in imaged as a bright spot, is equal to the reflector diameter. Therefore, in an f:1 aperture, the total included angle of the beam is approximately 90.degree.. Due to the greater distance from the source to the rim of the reflector where the image magnification is the least, the reflector lamp produces the smallest, most intense light spot almost entirely from the widest angle rim rays.
However, most graphics projectors have an aperture of at least f:4, where the maximum light acceptance angle of an image-projecting lens is only about 50.degree.. Thus, most of the lamp light, which is contained in the sharply-focussed light incident on the medium at between 50.degree. and 90.degree. off axis cannot be seen by the projection lens and is just thrown away.
In the case of optical fiber arrays the numerical light acceptance aperture is about f:2, as any incident light beyond the angle of total internal reflection of the fibers simply passes out the sides of the fibers and is lost as "side glow". Thus, the lamp light contained in the sharply-focussed light incident on the medium at between 70.degree. and 90.degree. off axis is also thrown away.
A first purpose of the present invention is to provide a projector having high-efficiency light coupler that absorbs and dissipates the beam heat before it reaches the projection medium. A second purpose of the present invention is to provide a projector in which sharply-focussed energy of an f:1 aperture projection lamp is accepted by the entrance end of an f:1 aperture coupler, and be emitted from the exit end of the coupler at an exit aperture of f:2 to f:4, or the optimum f-stop aperture for the specific graphic medium used in the projector.
A third purpose of the present invention is to provide a projector having a light coupler in which the sharply-focussed circular light pattern from a projection lamp is efficiently transformed by the coupler into a rectangular exit beam.
A fourth purpose of the present invention is to provide a projector having a light coupler in which wide angle rays from a projection lamp are substantially collimated into narrow angle rays that can be effectively utilized by the graphics medium.